In the UK it's normal practice to log 5 minute increments. Civil pilots log chock to chock, whilst military pilots log take-off to landing. [Used to really p&*^ us of at BDN that when doing a ground handling trial.] The exception is glider pilots, who have too many 2 or 3 minute flights to make 5 minute increments sensible.
Frankly, I think that if you've got enough hours to make a significant error, does it really matter if you have 150 or 160 hrs P1 (or 2000 or 2100) in a particular type? I try to keep my logbook as accurate as possible (and incidentally, most scientific calculators will add up hours and minutes) but don't lose any sleep over it.
G